1. Field of the Invention
The present invention relates to focus adjustment control of an imaging apparatus.
2. Description of the Related Art
An imaging apparatus is known to include an autofocus (hereinafter denoted as AF) function in which focusing is automatically performed relative to a subject. For example, there is a method in which an AF evaluation value is obtained by use of a band pass filter by extracting high frequency components from imaging data acquired by an imaging element through a lens, and the focus lens is moved to a peak position of the maximum AF evaluation value. The focus lens position at which the AF evaluation value is a maximum is the so-called just focus position. A method of aligning the focus lens to that position includes a calculation method or the like based on a variation amount or inclination amount in the AF evaluation value, or the increase of the AF evaluation value.
On the other hand, the effect of chromatic aberration on a lens, that is to say, the effect due to different focal distances resulting from the wavelength has been indicated in relation to an imaging apparatus. For example, when imaging by use of three primary colors of R (red), G (green) and B (blue), when focusing is performed by alignment with the G component, an image will result that is tinged with red on the edge portions of the subject. When a color appears on the edge portions of the subject as a result of chromatic aberration where that color was not originally present, a reduction in quality will result. Furthermore chromatic aberration causes a divergence in the focal position due to AF. This is due to the difference between the appearance to the human eye from the alignment of the focal position resulting from contrast.
In an imaging apparatus exhibiting chromatic aberration in the lens, the focal position aligned with the G component for example in an image that exhibits chromatic aberration appears to the human eye to be at the just focus position. On the other hand, when using a method in which an AF evaluation value is generated by use of contrast in the subject (color contrast) (that is to say, a contrast AF), the contrast difference in the subject image is greatest at the focus lens position at which the focus lacks sharpness and chromatic aberration is not present. Therefore, when there is an erroneous determination that this position is the just focus position, the captured image will be a blurred image. As a result, various methods have been proposed to avoid generation of blurring due to a chromatic aberration. The apparatus disclosed in Japanese Patent Laid-Open No. 8-289191 looks up a table stored in a memory when the zoom amount is varied as a result of displacement of the zoom lens, to thereby read out a focus lens position for each color corresponding to the zoom amount. The height varying unit of the respective imaging elements is driven independently in accordance with the read-out focus lens position so that the focus is corrected. The correction of blurring of focus is based on a reference table that contains an aberration correction amount respectively for RGB corresponding to the zoom lens position. Japanese Patent Laid-Open No. 1-128688 discloses another method of calculating a correction amount for any one of RGB and correcting the position of another imaging element in response to the calculation result.
The conventional techniques disclosed in Japanese Patent Laid-Open No. 8-289191 and Japanese Patent Laid-Open No. 1-128688 enable control to stop the AF operation at the just focus position by correction of the deviation of the focal position resulting from chromatic aberration. However, although focused state is enabled as a result, when coloring occurs on the edge portions of the subject image due to the effect of chromatic aberration, there is the possibility of an adverse effect on image quality.